Unaudited Quarterly Financial Data - Sales Prices for Shares of Common Stock (Details) - $ / shares |
Dec. 31, 2017 |
Sep. 30, 2017 |
Jun. 30, 2017 |
Mar. 31, 2017 |
Dec. 31, 2016 |
Sep. 30, 2016 |
Jun. 30, 2016 |
Mar. 31, 2016 |
|---|---|---|---|---|---|---|---|---|
| Maximum | Common Class A | ||||||||
| Class of Stock [Line Items] | ||||||||
| Sales price (in usd per share) | $ 15.40 | $ 16.55 | $ 17.18 | $ 16.48 | $ 13.50 | $ 11.16 | $ 9.80 | $ 10.00 |
| Maximum | Warrants | ||||||||
| Class of Stock [Line Items] | ||||||||
| Sales price (in usd per share) | 2.49 | 3.04 | 3.57 | 3.00 | 1.98 | 1.38 | 0.36 | 0.40 |
| Minimum | Common Class A | ||||||||
| Class of Stock [Line Items] | ||||||||
| Sales price (in usd per share) | 11.00 | 13.00 | 14.93 | 12.75 | 10.67 | 9.72 | 9.50 | 9.50 |
| Minimum | Warrants | ||||||||
| Class of Stock [Line Items] | ||||||||
| Sales price (in usd per share) | $ 1.23 | $ 1.69 | $ 2.43 | $ 1.73 | $ 1.13 | $ 0.25 | $ 0.16 | $ 0.21 |